Heating of hydrocarbon fluids



Dec-21,1948. L. KNEL mL 2,456,786

HEATING OF HYDROCARBON FLUIDS Filed May 18, 1945 3 Sheets-Sheet 1 Dec. 21, 1948 KNIEL ET AL y G oF HYDRocARBoN FLUIDS HEATIN 3 Sheets-Sheet 2 Filed May 18, 1945 IN V EN TORS 3 Sheets-Sheet 3 L. KNIEL..` ET AL HEATING oF HYDIOCARBON FLUIDs Dec. 21,' 1948.

Filed May 18, 1945 INVENTORS Q Knie!) rm mJy/lj;

o.. MO awo @Ho o...o DMU. and oHG QHGMU wenn U 4. 4 S 1 All s 1/ Patented Dec. 21, 1948 2,456,789; l HEATING oF mmnocAnoN FLUIns Ludwig Knlel, Scarsdnle, and Pierre Lambert, Jackson Heights, N. Y., and Herbert B. Treat, Teaneck, N. J., asslgnors to The Lummus Company, New York, N. Y., a corporation of Dela- Wart appucation May 1s, 1945, serial No. 594,410

1 'I'his invention relates to improvements in the `practice of heating a hydrocarbon fluid by passage of the iiuid through banks of tubing within a furnacefchamber and wherein the uid is heated predominantly by radiant heat. particularly the invention relates to heatinglof the fluid in accordance with said practice in an improved manner to effect pyrolysis or cracking of the iiuid.

An important'object of the invention is to pro- .vide for increased thermal eiiiciency in the heating of the iiuid to a cracking temperature and also to effect economy in the heating equipment.

Another object of the invention is to provide for substantial reduction of coke deposition within the cracking tubing and thereby materially in,- crease the heat transfer efliciency of the tubing 'and also avoid frequent shut-down of theheating apparatus for removal of the coke;

Another4 object of the invention is to obtain said' reduction in coke deposition in a simple and comparatively inexpensive manner by the provision of an improved arrangement of the tubing and the heating means for more even distribution of heat around the cracking tubing.

Another object of the invention is to provide for eilicient preh'eating of the fluid by direct radi` ant heat and/ionthenv raising the temperature of the fluid to a cracking point by the application of direct radiant heatI reflected radiant heat and heat obtained by convection iiow of yproducts of combustion vwithin the furnace chamber, said combined heat being applied in a manner to minimize coke deposition.

`ing description taken with the drawings.

'In the drawings: Fig. 1 Yis a vertical transverse sectional view of a furnace embodying our invention, a portion of 'an adjoining similar furnace alsol appearing in the view;

Fig. 2 is a vertical Fig. l;

Fig. 3 is an enlarged detail sectional view on the line 3;-3 of Fig. 2;

section on the line 2-#2 of Fig. 4 is a view similar to Fig. 1, showing theinvention. embodied in a furnace of modified form; and A b 'l Fig. 5 is a top plan view, uponl areduced scale, of the furnace construction shown in Fig. 4.

More l 13 claims. (ci. 19e- 110) 2 Our invention is of articular utility in the thermal cracking'oi' pr pane to obtain ethylene as a desired end product. In such' cracking the propane charge. under approximately atmosphe'ric pressure. must be heated to a temperature l of approximately 1400 F. Under that high temperature a substantial amount of coking occurs and, in furnaces of conventional design, much oi' the coke is deposited within the heating tubes,

with consequent impairment oi thermal emciency and the necessity of frequent shut-down of the furnace to remove the coke Vfrom the tubes. The coke deposition is due mainly to unequal'distribution of the applied heat around the circumference of the tubes. It has been found that the coke passes to the colder side of the tubes and is deposited there instead of being Ycarried through the tubes by the charge stream. The present invention provides, along with other advantages,. for substantially even distribution of applied heat around the cracking tubes and thereby substantially reduces the amount of coke deposited within the tubes. of propane is specifically mentioned the invention is well adapted for the heating of petroleum crude or various distillates thereof to effect distillation or cracking. j K

According to the invention the charge to be cracked is first passed progressively upward through a vertical bank of tubes disposed ad-` jacent a side wall of the furnace chamber and the tubes of this bank are heated predominantly by direct radiant heat. Thereby, the charge is quickly preheated but its temperature is not raised suiciently for cracking or coking of any portion of the charge. The charge, so preheated, is next passed progressively downward through a second-stage bank of heating tubes which are heated evenly'around their entire circumference predominantly by direct radiant heat but with the addition of reflected radiant heat and convection heat obtained by inducing a flow of hot combustion products over the upper tubes of the bank.` Thus the temperature of the charge is quickly raised to the cracking point and, at the same time, coke deposition is minimized by proper distribution of the heat.

Referring to\the embodiment of the invention shown in Figs. 1 and 2 of the drawing, the furnace chamber I is of box-likeform, having a flat iioor or hearth 2, a iiat roof 3, vertical end walls l and! and opposite side walls 6. The lower portions of the side walls are vertical and the upper portions converge upwardly to the roof. The chamber is constructed oi' blocks or slabs of While the cracking assenso 3 refractory material sheathed with heat insulation 1. The furnace is supported in a position elevated from the ground or from a foundation by a metal frame structure comprising columns 8. and upper and lower girders 9 and I0, respectively. One or more similar furnaces may also besupported by said frame structure, a portion of an additional furnace being shown in Fig. 1.

Within the furnace chamber a pair of outer tube banks II and I2 are supported in vertical positions directly inwardly of the vertical porpositions between said outer banks. These intermediate banks are arranged parallel to the outer banks and are preferably equally spaced from each other and from the adjacent outer banks. A pair of similarly spaced, relatively low walls I5 extend upwardly from the floor of the furnace l the row and directed upwardly through ports in the floor. All of the burners are individually adjustable to vary the heat input in the three zones. 'I'he roof of the furnace chamber has an outlet I8 opening from the central zone B into an exhaust duct I9 for discharging products of combustion from the furnace chamber. The out'- let I8 and the duct preferably extend substantially the length of the furnace chamber, and the duct has a damper for adjusting the draftI through the furnace.

Each of the four tube banks comprises a singleI tions ofthe side walls 6. A pair of intermediate ytube banks I3 and I4 are supported in vertical tions adjacent the right hand side of the furnace chamber. These closures are formed at their inner sides of refractory material.

'I'he tracks 28 are in the form `of I-beams and I the suspension connections include rollers 29a to travel upon the lower flanges of the beams.`

Yokes 29h bear said rollers and are pivotally connected, on axes transverse to the plane of the tube bank, to depending links 29e. At their lower ends said links are pivotally connected, on axes extending longitudinally of the tube banks, to vertical bars 32 which support the tubes of the bank in spaced relation. The lower ends .of these bars project into a channel 33 in the upper edge of the underlying wall I5, to steady the bank. Metal wear strips 33a of U form are fitted in the channel in positions to straddle the lower ends of the bars 32 when the bank is in operative position within the furnace chamber and prevent wearing contact of said bar ends with the refractory material of the wall I5. These wear strips are secured to the upper end of the metal structures I6 of said wall. A stay rod 29d connects together the links 29e of all of the suspension connections of the tube bank. and-a stay rod 32a connects together the Abars 32 adjacent the lower ends of vertical row of spaced horizontal tubes'all in a y common vertical plane and serially connected by return bends for now of the charge in a serpentine course through all of the tubes in-succession. Each of the outer tube banks has an inlet '2| at its lower end connected to a charge line 22, as shown in Fig. 2. At its upper end, each outer bank has an outlet 23. Through a detachable tube connection 24, said outlet is in delivery connection with the inlet 25 of the adjacent inter-f mediate bank, at the upper end of the latter. Each intermediate bank has at its lower end an outlet 28 connected to a discharge line 21. All of the banks, including their return bends, are conned within the furnace chamber and extend throughout most of the length of the chamber.

I The tube of the intermediate banks are subjected to greater heat than the tubes of the outer banks and they consequently require more frequent servicing and replacement. Therefore, provision is made for ready removal of the intermediate banks individually. The frame structure which supports the furnace bears a pair oi.' horizontal tracks 28 extending over the top of the furnace and outwardly materially beyond the wall 5` of the furnace. These tracks lie in the vertical planes of the intermediate tube banks, and through a plurality of suspension connections 29, the banks are individually supported by the respective tracks. dThe suspension connections extend through slots 3a in the roof 3. With reference to Fig. 2, these slots extend inwardly from the right-hand side of the furnace chamber but terminate short of the opposite side w'all 4.

said bars. The channel 33 of each wall I5 has an end wall 33h in a position to be engaged by the lower ends of the bars 32 which are nearest the wall 4 of the furnacechamber and thereby determine the proper longitudinal position of the intermediate tube banks.

. The end wall 5 of the furnace chamber has a, pair of vertically elongated apertures or slots 34 35 the tube banks I3 and I4 may be pulled indi- `through in parallel.

vidually along their respective tracks outwardly through the slots 34 to the exterior ofthe furnace chamber where they may be conveniently serviced. V

.Inthe operation of the furnace just described the charge to be heated is continously introduced into the lower ends of the outer tube banks I I and I2 andpassed progressively upward there- Within said banks the charge is quickly preheated, predominantly by radiant heat including that received directly from combustion within the zones A and C and that received through reradiation from the walls I5. While the tubes of these banks are heated predominantly upon one side thereof there is no danger of coke deposition since the charge is cold when it enters the banks and it does not remain therein long enough to reach a coking ttemperature. Cljhe provision of these banks avoids the requirement for the customary convection heating -bank and the chamber containing such a bank and effects more eflicient and quick pre- In the flow of the charge through the tubes th portion of the charge stream nearest theY tube walls flows more slowly than the central portion of the stream and becomes heated to a greater degree than said central portion due to the time factor and due also to its nearness to the heated tube walls. In the cracking of a propane charge. for example, the outer portion of the charge stream may be cracked to form undesired acetylene. The preheating tube-banks are arranged to prevent such excessive cracking. They are arranged so that they will absorb much direct radiant heat which would otherwise go to the cracking tubes and so that they will also blanket areas of the walls of the furnace chamber which would otherwise reflect radiant heat upon the lower ones of the cracking tubes. Thereby, the application of cracking heat to such tubes is desirably l limited.

The preheated charge is passed from the upper ends of the banks II and I2 through the connections 24 and 25 to the upper ends of the intermediate tube banks I3 and I4, respectively. In these banks the charge passes progressively downward, in parallel, tothe discharge lines'connected to the lower ends of the banks. In'its passage through the intermediate banks the charge is quickly raised to the required high temperature for cracking by both radiant heat and heat of convection. The heating is predominantly by radiant heat and such heat is yapplied to the tubes substantially evenly around their circumference so that, coke deposition is very greatly reduced. Since the tubes of the intermediate bank I3 are exposed to the combustion zones A and B at opposite sides thereof and the tubes of the bank I4 are exposed to the zones B and C the tubes of both of said banks receive radiant heat in substantially even. distribution around their circumference. The radiant heat received includes that directly radiated from combustion in all of said zones, that received by reflection from the inclined upper portions 6a of the side walls 6 and that received by reradiation from the walls I5 and from other exposed wall surfaces of the furnace chamber. A substantial amount of radiant heat is received by reflection from said wall portions 6a which are disposedto reflect the heat predominantly upon the upper portion of the banks. These wall portions are of large area so that they compensate for loss of heat reflection from the lower portions of the walls 6 due to the blanketing effect of the outer tube banks II and I2'. At the same time, the reflected heat is directed predominantlyupon the upper tubes of the cracking banks instead of upon thelower tubes thereof, to avoid excessive cracking within the lower tubes.

Another important thermal advantage is obo tained by provision for obtaining effective application of heat of convection to the tubes of the intermediate banks. vSince the products of combustion from all of the zones A, B and C are discharged from the top of the centrazone there is obtained a sweep of the gaseous combustion products from all of said zones over the banks I3 and I4 and through the intertube spaces thereof from the zones A and C to the outlet I8. A quite substantial amount of heat is effectively applied to the charge in this manner, and such provision for convection heating is an important feature of our invention.

Figs. 4 and 5 show a modification of our Improved furnace construction. This modification provides for exact duplication of radiant heating yconditions at opposite sides of an intermediate tube bank. The furnace chamber, designated Ia.

,is made more narrow and is provided'with a single intermediate tube bank 40 and a'single low wall I5a, together dividing the chamber into two combustion zones D and E of equal size. Connections 24a pass lthe charge from both of vthe outer banks II and I2 to the upper end of the central bank, and the tubes of the latter are of larger diameter than the tubes of the outer banks to accommodate the double charge received therefrom.

The zones D and E are provided with individual outlets 4I and 42 respectively, through the roof 3b for discharge of combustion products. y outlets open respectively into exhaust ducts43 and 44 which deliver into a common exhaust duct 45. Each of the ducts 43 and 44 is provided with a damper 46 to control exhaust therethrough. By adjustment of the damper in duct 43 to choke exhaust therethrough the combustion products from both zones will be compelled to escape through the outlet 42 and said products from zone D will be constrained to pass through the tube bank 40 to reach the exit 42. Similarly, by opening the damper in duct 4I and closing the damper in duct 42 the combustion products from both zones will be compelled to escape through the out# let 4I and such products from zone E will be constrained to pass through the tube bank 40 to reach the exit 4I. By either of said damper adjustments there is obtained effective application of heat of convection to the tubes of the central bank.

With the exception of the differences pointed out the construction of the furnace shown in Figs. 4 and 5 is similar to that shown in Figs. 1, 2 and 3. similar parts are correspondingly numbered.

' It is obvious that the refractory wallsa of Fgure 1 and Figure 4 will transmit heat by reradication as well as by reflection. In the specification and claims the word reflect is intended to comprehend reradiate also.

It is, of course, to be understood that the foregoing description is merely illustrative and in nowise limiting and that we desire tocomprehend within our invention such modifications as are included within the scope of the appended claims.

Having thus clearly described our invention, what we claim as new and desire to secure by Letters Patens is:

1. The method of cracking a hydrocarbon fluid` comprising conducting combustion predominantly within lower regions of each of two side-by-side zones, passing said fluid in a confined stream progressively in one vertical directiony along a serpentine course through an outer wall region within one of said zones and remote from the other zone, to preheat the fluid predominantly by radiant heat. passing said preheated fluid from said course progressively downward in a confined stream along a serpentine course leading from a point materiallyv higher than said first course downwardly between `said zones,.equally exposing opposite sides of said downward coi/irse to radiant heat from both zones and, in said downward course, further heating the fluid to a cracking temperature by direct radiant heat from both of said zones, by reflection of radiant heat upon an upper portion of the course and by discharge oi' the products of com ustion of one of said zonesacross an upper por; ion of the course.y

2. A heater for hydrocarbon fluid, comprising a furnace chamber, afpair of vertical and substantially fiat outer banksy of serially connected These horizontal tubes within said chamberand disposed respectively adjacent opposite side walls of the chamber, a pair of vertical and substantially iiat intermediate banks of serially connected horizontal tubes in transversely opposed spaced relation to each other and to said outer banks respectively and dividing the chamber into outer combustion zones and an intermediate combustion zone, each of said intermediate banks comprising a single row of tubes with the individual tubes of the row equally exposed transversely to the zones at opposite sides of the row, each of said outer banks having at opposite ends of the vertical length thereof an inlet and an outlet for the uid, each of said intermediate banks having an the lower end of said row of tubes a uid outlet and the outlet of each outer bank being interconnected with the upper end of the adjacent one of said intermediate banks, for passage of tw'o separate streams of the fluid through the outer banks and thence downwardly through the intermediate banks in parallel, and burner means in lower regions of said three zones to heat the outer banks predominantly by radiant heat from the outer zones respectively and to heat the intermediate banks `predominantly by radiant heatfrom the 'intermediate zone and from the outei' zones resaid chamber adjacent opposite side walls of the chamber and arranged substantially symmetrically with respect to said plane, a single vertical row of serially connected vertically spaced horizontal tubes located within said plane midway between said outer banks and extending between and materially'above the level of the latter and dividing the chamber into substantially duplicate combustion zones, each of said outer banks having an inlet and an outlet for passing iiuid to s be heated serially through the tubes of the bank, a delivery connection between the outlet of each of said banks and the upper end of said intermediate row of tubes for downward passage of the uid from both of said banks through the tubes of the row serially, the row having an outlet disposed for discharge of the iluid after said serial iiow, burner means disposed for fuel combustion within lower regions of said zones, the tubes of the intermediate row being individually exposed around substantially their entire circumference to receive radiant heat from the combustion within both of said zones simultaneously and substantially equally and said opposite side walls having portions thereof above saidouter banksand of substantial area inclined upwardly and inwardly toward the plane of the intermediate row of tubes to reflect heat predominantly upon an upper portion of saidrow, land outlet means disposed for passing products of combustion from the upper region of said zones.

4. A heater for hydrocarbon uid, comprising a -furnace chamber substantially symmetrical with respect to a vertical plane, a pair of substantially coextensive outer banks of vertically spaced ilo serially connected horizontal tubes within said chamber adjacent opposite side walls of the chamber and arranged substantially symmetrically with respect to said plane, a single vertical row of serially connected vertically spaced horizontal tubes located in said plane midway between said outer banks and extending between and materially above the level of thelatter and dividing the chamber into substantially duplicate combustion zones, each of said outer\banks having an inlet and an outlet for passing iid to be heated serially through the tubes of the bank, a delivery connection between the outlet of each of said banks and the upper end of said intermediate row of tubes for downward passage of the duid/from both of said banks through the tubes of the said row. serially, the row having an outlet disposed for discharge of the fluid after said serial ow, said outer banks and said intermediate rowof tubes being all spaced above the lowerlside of the chamber, burner means disposed for fuel combustion within both of said zones at points lower thany the tubes therein, the tubes of the intermediate row being individually exposed around substantially their entire circumference to receive radiant heat from the combustion within both of said zones simultaneouslyl and substantially equally, and said opposite side walls having portions thereof above said outer banks and of substantial area inclined upwardly and inwardly toward the plane of the intermediate row of tubes to reflect heat predominantly upon an upper portion of said row, and outlet means disposed for passage of products of combustion from the upper region of said zones.

5. A heater for hydrocarbon uid, comrising a furnace chamber substantially symmetrical with respect to a vertical plane, a pair of substantially coextensive outer banks of serially connected ver- 40 tically spaced horizontal tubes4 within said chamber adjacent opposite side walls of the chamber and arranged substantially symmetrically with respect to said plane, a single vertical row of serially connected vertically spaced horizontal tubes located in said plane midway between said outer banks and extending between and materially above the level of the latter and dividing the chamber into substantially duplicate combustion zones, each of said outer banks having an inlet and an outlet for passing fluid to be heated serially through the tubes of the bank and in the same vertical direction in both banks, a delivery connection between the outlet of each of said banks and the upper end of said intermediate row of tubes for downward passage of the iiuid from both of said banks through the tubes of said row serially, the row having an outlet disposed for discharge of the fluid after said serial fiow, said outer banks and said intermediate row of tubes being all spaced above the lower side of the chamber. burner means disposed for fuel combustion within both of said zones at points lower than the tubes therein, the tubes of the intermediate row being individually exposed around substantially their entire circumference to receive radiant heat from the combustion Within both of said zones simultaneously and substantially equally, and said opposite side walls having portions thereof above said outer banks and of substantial area inclined upwardly and inwardly toward the plane of the intermediate row of tubes to reflect heat predominantly upon an upper portion of said row, and outlet means disposed for passage of products of combustion 'from the upper region of said zones.

. an inlet and an outlet for passing fluid to be heated through the tubes of the bank, a delivery connection between the outlet of each of said outer Vbanks andthe upper end of said intermediate bank for downward passage of the iluid from both outer banks through the tubes thereof, the intermediate bank having an outlet disposed for Vdischarge of the fluid after passage through the bank, burner means disposed for fuel combustion 90 within lower regions of both of said zones, the

tubes of the intermediate bank being individually exposed around substantially their entire circumference to receive radiant heat from the combustion within both of said zones simultaneously and substantially equally, and said opposite sidejwalls having portions thereof above said outer banks and of substantial area inclined upwardly and inwardly toward the plane of the intermediate bank to reflect heat predominantly upon an upper portion of the latter, and outlet means'disposed for passage of products of combustion from the upper region of said zones.

'7. A heater for hydrocarbon-uid, comprising a furnace chamber substantially symmetrical with respect to a plane, a pair of outer banks of tubes adjacent opposite sidewalls of the chamber and arranged substantially symmetrically vwith respect to said plane, an intermediate tube bank comprising a single row of transversely spaced 40 tubes arranged within said plane midway between the outer banks and dividing the chamber into substantially duplicate combustion zones', said intermediate bank having a substantial portion thereof extending materially beyond the outer banks and each of the latter having an inlet and an outlet for passing the fluid to be heated through the bank, a delivery connection between the outlet of each outer bank and the outer end of said extending portion of the intermediate bank for passage of the fluid from both outer banks in one general direction from said end ofthe intermediate bank through the tubes of said row. the row having an outlet disposed for discharge of ther' fluid after passage thereof through the tubes of the row, burner means arranged for combustion of fuel within regions of both of said zones remote from said extending portion of the intermediate bank, the tubes of said interm diate row being individually exposed around su stantially their entire circumference to receive radiant heat from the combustion within both of said zonesI simultaneously and substantially equally, said sidewalls having portions thereof of material area located at'opposite sides of said extending portion of the intermediate bank and inclined toward said plane to reflect heat predominantly upon said portion of the bank, and outlet means disposed for passage of products of combustion from regions of said zones adjacent said extending portion of the intermediate bank,

8'. A heater as claimed in claim 7 wherein the said combustion products outlet means includes a separate outlet for each of said zones and a damp- 76 er for each outlet, said dampers being selectively operable.

9. The method of heating a. hydrocarbon fluid to a conversion temperature, comprising passing such uid in a pair of confined streams respectively along a pair of transversely opposed spaced serpentine preheating courses, conducting the fluid from both of said courses in a single coniined stream along'a serpentine conversion course from apoint materially beyond the space between said preheating courses into said space and midway between the preheating courses, conducting combustion within the spaces between said conversion course and the preheating course, heating the fluid within said preheating courses predominantly by radiant heat from said combustion, equally exposing opposite sides of the conversion course simultaneously to radiant heat from said combustion within both of said spaces, reflecting radiant heat of said combustion predominantly upon opposite sides of the portion of the conversion course extending beyond the space between preheating courses, heating the fluid within the conversion course to a conversion temperature predominantly by said radiant heat received from opposite sides of the course and limiting the conversion in the advanced portion of the' course by the absorption of heat by the uid within the preheating courses.

a furnace chamber substantially symmetrical with respectto a vertical plane, a pair of substantially coextensive outer banks of serially connected vertically spaced horizontal tubes within said chamber adjacent opposite side walls of the chamber and arranged substantially symmetrically with respectto said plane, a single vertical row of serially connected vertically spaced horizontal tubes located within said plane and extending between said outerbanks and dividing the chamber into substantially duplicatecombustion zones, each of said outer banks having an inlet and an outlet arranged for passing uid to be heated serially through the tubes of the bank and in the same vertical direction in both outer banks simultaneously, a delivery connection between the outlet of each outer bank and the upper end of said intermediate row of tubes for downward passage of the uidfrom both outer banks through said .row of tubes, upwardly directed burners spaced lengthwise of the tubes of said row disposed for fuel combustion within lower regions of said zones, between the outer banks and said row of tubes, the tubes of said inter'medlate rows being individually exposed around substantially their entire circumrference to receive radiant heat from both of lsaid zones simultaneously and substantially equally, and outlet means `disposed'for passing products of combustion'` from the upper regions of said zone.

11. The method of heating'a hydrocarbon fluid, comprising conducting combustion predominately within lower regions of two side-by-side zones, passing said uid in two separately confined streams and in the same general vertical direction along two substantially coextensive serpentine courses at the opposite outer sides of said zones to preheat the iluidpredominantly by radiant heat, combining said streams after passage through said courses, and passing the fluid of the combined streams progressvely downward in a confined stream along a serpentine course between said zones and substantially midway between said first courses and, in said downward course,7 applying radiant heat from both of said zones simultaneously and substantially. equally to opposite 10. A heater for hydrocarbon uid, comprising l' sidesoithe crosssectionotthecoursetoiurther heat the iluid.

12. The method of heating -a hydrocarbon duid -to a conversion temperature, comprising conducting combustion predominately within lower regions on two side-by-side zones. passing said lluid in two separately confined streams and' in the same general vertical directionl along two substantially coextensive serpentine courses at l the opposite outer sides of said zones to preheat the fluid predominantly by radiant heat, combining said streams alter passage through said courses, passing the fluid of the combined streams progressively downwardV in a conilned stream along a serpentine conversion course leading from a point materially higherthan said first courses downwardly between said zones and substantially midway between said ilrst courses and, in said downward course, applying direct radiant heat from both oi said zones simultaneously and substantially equally to opposite sides oi the cross section of the course, and reflecting radiant heat predominantly upon opposite sides of an upper portion of the course.

'13. A heater for hydrocarbon iluid, comprising a furnace chamber substantially symmetrical with respect to av central plane, a pair oi' substantiallyv f coextensive outer blanks o! serially connected transversely spaced tubes .within said chamber adjacent opposite side walls of the chamber and arranged substantially symmetrically with respect to said plane, a single row of serially connected transversely spaced and substantially parallel tubes located within said plane and extending between said outer banks and dividing thel chamber into substantially duplicate combustion zones, each of said outer banks having an inlet and an outlet arranged for passing iluid to be heated serially through the tubes of the bank and in both banks simultaneously, a delivery connec- 40 tion between the outlet of each outer bank and' one end of said intermediate row of tubes for`pas sage of the iluid from both outer banks through I the tubesot said row serially, burners spaced lengthwise ot the tubes ci' said row and directed toward the duid-receiving end of the row and disposed for fuelcombustion predominantly within regions of the zones adjacent the opposite end of the row and between therow and the outer banks,

the tubes of saidv row being individually exposed 1 around substantially their entire circumference to receive radiant heat from both zones simultaneously and substantially equally, and outlet means disposed for passing products of combustion from regions of said zones adjacent the iluidreceiving end of said row.

LUDWIG KNIEL. PIERRE LAMBERT. HERBERT R. TREAT.

REFERENCES CITED The following references are of record in the file of thisipatent:

I UNITED STATES PATENTS Date 

